There exists a class of devices for accessing fluid spaces and vessels within a human (or animal) body that are generally referred to as "catheters". Herein, "vessel" is defined as any conduit carrying a fluid within the patient's body. These prior art devices comprise flexible tubes of fluid-impermeable material, and are often provided with functional apparati suited to various purposes, such as attachment, flowpath regulation (i.e., valving), etc.
Catheters are an essential component of any fluid exchange therapeutic device. Although it is known to percutaneously implant one or two catheters into a patient, thereby allowing repeated access to the patient's circulatory system, the high incidence of infection, stenosis, patient discomfort, and site/device failure has led to a search for a dual-lumen catheter that provides effective and reliable longterm operation.
A primary method of performing fluid exchange therapy, such as hemodialysis or therapeutic apheresis uses a dual-lumen catheter. The catheter is placed partially with the body with the distal end placed in a blood vessel and the proximal end outside the body. The proximal end is connected to a blood processing machine. One lumen of the catheter is for removing blood from the vessel and bringing it to the machine, and the other lumen returns blood that has been processed back to the body.
Low fluid resistance and low flow velocity in the laminar flow regime (rather than turbulent flow) are desirable because these glow attributes lessen damage to cellular components in the blood. To reduce fluid resistance, flow velocity, and maintain laminar region with high flow rate, which is now the trend, requires the lumen diameter to increase. However, practical limits exist because of the limited space within the blood vessels containing the catheters. As one increases the outside dimensions of the catheter, a reduced flow area is left for flow of blood within the vessel. This leads to blood flow disturbances within the vasculature and can lead to stenosis and thrombosis within the blood vessels. Reduced flow area can also result in an inability to withdraw the prescribed amount of blood required for effective hemodialysis therapy. Patient tolerance and discomfort are also significant considerations.
U.S. Pat. No. 5,106,368, entitled "COLLAPSIBLE LUMEN CATHETER FOR EXTRACORPOREAL TREATMENT" and issued 21 Apr. 1992 to Uldall et al. (the '368 patent), shows a dual-lumen catheter with a double-barrel shotgun configuration when disposed in vivo. A first lumen is defined by a relatively thicker wall, while a second lumen is defined by a relatively thinner wall that is collapsible against the thicker first wall for ease of insertion into a blood vessel. This insertion is accomplished by collapsing the relatively thinner wall of the catheter against the relatively thicker wall so that the entire catheter may be disposed within a peel-away sheath having a cross-sectional area smaller than that of the expanded catheter.
Uldall et al. disclose that this thinner wall is able to withstand the positive pressure required for fluid return and, further, the negative pressure required for fluid withdrawal, allowing the flows to be reversed without collapsing the lumen. Thus, once the '368 catheter is inserted, it retains its fully expanded shape and does not experience lumen collapse, during either dialysis or latency.
The overall cross-sectional shape of the Uldall et al. catheter is either "figure-8" shaped or ellipsoid. However, because these lumens must each have a sufficient cross-sectional area to accommodate fluid flowrates of up to 500 cc/min, the catheter unavoidably must have a significant external cross-sectional diameter, leading to the same adverse effects described above.
Another dual-lumen catheter is disclosed in U.S. Pat. No. 5,380,276, entitled "DUAL LUMEN CATHETER AND METHOD OF USE" and issued 10 Jan. 1995 to Miller et al. (the '276 patent). The '276 patent describes a dual-lumen catheter tube having two coaxial lumens defined by a substantially circular outer wall member separated by a substantially circular inner common support wall that joins the outer wall member. One lumen is substantially circular in cross-section, while the other is crescent-shaped in cross-section and substantially surrounds the first lumen. While the coaxial configuration of the Miller et al. '276 device addresses the known problems associated with a large diameter cross-sectional shape, the construction chosen presents other problems not seen in other devices.
First, the walls of the '276 device are of equal thickness. Miller et al. disclose that the crescent-shaped lumen may be used for removing blood from a dialysis patient and that the circular lumen may be used for blood return to the patient. This arrangement will not work in practice. The negative pressures created by dialysis machines necessary to remove blood at the rate of 500 cc/min would immediately cause the outer wall defining the crescent-shaped lumen to collapse against the inner support wall.
Miller et al. do disclose that the device may be used in a reverse manner, with fluid withdrawal accomplished via the circular lumen and fluid return via the crescent-shaped lumen. However, in a device meeting the dimensions required by the '276 disclosure, even this arrangement would result in the immediate collapse of the circular lumen, due to the same large negative pressures created during dialysis. Further, if the walls of the device were thickened so that such pressures would not collapse the respective lumens, the overall diameter of the catheter would be outside the recited ranges and the benefits of the coaxial structure vis-a-vis other known structures would be forfeited. Alternately, if the walls were constructed of a material sturdy enough to withstand these pressures, the catheter would be unacceptably stiff, resulting in a range of complications, including access site trauma, infection, and patient discomfort.
The main problem common to known twin catheters or double-lumen catheters introduced into the internal jugular vein (which is now the preferred entry point for hemodialysis) is that they occupy a large cross-sectional area in the vein. Thus the maximum outer diameter of the catheter is limited, so as to avoid an increase that is unacceptable in blood shear rate in the vessel containing the catheter. As a result of the limitation on the outer catheter diameter, the catheter lumen diameter is also limited. However, limiting the catheter lumen diameter often results in stenosis, thrombosis, platelet activation, and other problems associated with bloodflow through the catheter. Such limitations also limit the fluid flows possible, thereby limiting efficiency and reducing the possibility of decreasing treatment time.
Accordingly, it is an object of this invention to overcome the above illustrated inadequacies and problems of extant dual lumen catheters by providing an improved dual lumen catheter suitable for repeated use in applications (e.g., hemodialysis) requiring blood flowrates of 250-500 ml/min or greater.
It is another object of this invention to provide a dual-lumen catheter wherein one lumen is collapsed due to physiological pressure when the catheter is not in use.
A still further object of the present invention is to provide a dual lumen catheter that does not present an increase in blood shear rate, thereby avoiding stenosis, thrombosis, platelet activation, and other problems.
Yet another object of the present invention is to provide a dual-lumen catheter having lumens of sufficient cross-sectional area so as not to limit the efficiency of hemodialysis therapy and reduce patient treatment time.